This invention relates to a drive system for construction machinery, such as a hydraulic shovel, hydraulic crane, etc., and a method of controlling hydraulic circuit means of such drive system.
Heretofore, a drive system for construction machinery, such as a hydraulic shovel, hydraulic crane, etc., comprises an open-type hydraulic circuit having a plurality of hydraulic actuators receiving a supply of pressurized fluid for actuating a boom, arm, bucket and other movable members of the machinery connected thereto.
The present practice for controlling the speed of each movable member is to adjust the opening of a hydraulic directional control valve associated therewith. More specifically, this control system comprises a variable resistance provided in a hydraulic circuit by adjusting the opening of the hydraulic directional control valve, so that a loss of energy is produced in the variable resistance for controlling the speed of the hydraulic actuator. Thus, total efficiency is essentially reduced when this control system is used. With a view to operating construction machinery at a reduced energy consumption level, there have, in recent years, been many attempts made for providing improvements in the art of construction machinery. One example of such attempts is disclosed in "Olhydaulik und Pneumatik", pages 213-222, April 1976 number, which proposes to optimize control by pumps for improving the performance of excavators. The proposal contemplates connecting variable-displacement hydraulic pumps to hydraulic actuators in closed or semi-closed circuit, to control pump deliveries for controlling actuator speeds. In this closed-type hydraulic circuit system, the hydraulic pumps are only required to generate necessary power, and the energy of gravity or the energy of inertia acting on the hydraulic actuators can be absorbed by the engine via the hydraulic pumps. This enables total efficiency to be markedly increased. This system includes, in its concrete construction, a plurality of hydraulic pumps each connected in closed circuit to one or two hydraulic actuators. It is shown that when this hydraulic circuit was used for operating a hydraulic shovel, there was no need to use oil coolers until the ambient temperature has risen to 25.degree. C. and that even in midsummer when ambient temperature rises to 40.degree. C. the temperature of oil did not rise above 70.degree. C. with the use of oil coolers of a capacity which is about one half that of those of the prior art. Thus, the proposed system has been proven to have effect in economizing on energy consumption.
Although the proposed closed-type hydraulic circuit has been proven to have effect in economizing on energy consumption, various problems must be obviated before it can have actual application in construction machinery. One of such problems is how to effect matching of the hydraulic pumps and hydraulic actuators in capacity. For example, in this hydraulic circuit, one hydraulic pump is connected to a boom cylinder and a travelling motor for selectively driving one of them. In this case, it is not in the interest of efficiency to design to hydraulic actuator to have a capacity such that the maximum flow rate for the boom cylinder and the maximum flow rate for the travelling motor have the same value. The boom cylinder is required to generate a high thrust and also to act at high speed. Because of this, the boom cylinder is required to have a large pressure receiving area, and it is sometimes necessary to supply fluid in high flow rate to the boom cylinder. Thus, the hydraulic pump should have a high capacity to supply fluid to the boom cylinder in an amount that satisfies its need, but it is not economical for the hydraulic pump to have a high capacity which is too high for operating the travelling motor. This is also true of the hydraulic circuit wherein a single hydraulic pump drives an arm cylinder and another travelling motor.
Another problem raised with regard to this hydraulic circuit is that limitations are placed on simultaneous operation. For example, the pressurized fluid from the hydraulic pump is used for operating the travelling motor during travelling so that neither the boom nor the arm can be operated.